Various aluminate phosphors have been put to practical use as phosphors for energy-saving fluorescent lamps. As a blue phosphor, for example, (Ba,Sr)MgAl10O17:Eu (hereinafter referred to as “BAM:Eu”) is mentioned. As a green phosphor, for example, CeMgAl11O19:Tb, BaMgAl10O17:Eu,Mn, and the like are mentioned. In recent years, BAM:Eu, which has high luminance under vacuum-ultraviolet excitation, has been used as a blue phosphor for various commercially available PDPs.
However, when a PDP, especially one using the blue phosphor BAM:Eu, is driven for a long period, the luminance is degraded significantly. Hence, in the PDP use, there is a strong demand for a phosphor that shows less luminance degradation even after the long-time driving.
In response to this demand, JP 2003-132803 A, JP 2004-176010 A, and “Excitation and Luminescent Characteristics of (Sr,Ba)3MgSi2O8:Eu2+Blue Phosphors” (IEICE technical report, The Institute of Electronics, Information and Communication Engineers, Vol. 105, No. 568, pp. 53-56) propose methods using silicate phosphors such as Sr3−eMgSi2O8:Eue (where 0.01≦e≦0.1) and (Sr1−y,Bay)3−fMgSi2O8:Euf (where 0≦y≦1 and 0.01≦f≦0.1). “Excitation and Luminescent Characteristics of (Sr,Ba)3MgSi2O8:Eu2+Blue Phosphors” also proposes a technique for adjusting color purity by replacing part of the Sr sites in a silicate phosphor Sr3−eMgSi2O8:Eue with Ba. JP 2006-12770 A also proposes a use of M3−gMgSi2O8:Eug (where M is at least one element selected from a group consisting of Sr, Ca and Ba, and 0.001≦g≦0.2).
The silicate phosphor Sr3−eMgSi2O8:Eue is proposed as a phosphor that shows less luminance degradation even after the long-time driving when it is used for a PDP. However, it has a problem that the chromaticity y is higher and the color purity is worse than those of a blue phosphor BAM:Eu that is currently used in the PDP.
On the other hand, the color purity can be adjusted to a level comparable to that of BAM by replacing part of Sr with Ba so as to obtain a silicate phosphor represented by (Sr1−y,Bay)3−fMgSi2O8:Euf. Yet, such a phosphor has a problem of the luminance being lowered. For example, the above-mentioned non-patent document 1 reports that the emission peak intensity of the phosphor is lowered to about 60% to 70% of that of the commercially available BAM:Eu phosphor.
Thus, blue silicate phosphors having both good luminance and chromaticity y have not been obtained so far.